1. Field of the Invention
The present invention relates to a liquid supply apparatus that performs printing, coating, or the like by supplying liquid, such as ink or varnish, with a liquid supply member, through holes formed in a plate, such as a screen, to a sheet, such as a paper sheet, held by an impression cylinder and a method of manufacturing a plate therefor. Particularly, the liquid supply apparatus of the present invention is effective when applied to a screen printing unit of a printing press that performs screen printing on a paper sheet.
2. Description of the Related Art
For example, in a conventional procedure of screen printing on a paper sheet, a rotary screen apparatus is employed. The rotary screen apparatus has a cylindrical thin screen in which small holes corresponding to a pattern are formed, and is provided with a squeegee which is a liquid supply member placed inside the screen. Liquid such as ink or varnish, which is stored inside the screen, is squeezed out through the small holes of the screen by the squeegee. In this way, a screen printing corresponding to the pattern is performed on a paper sheet held on an outer circumference of an impression cylinder by using the liquid such as ink or varnish. Since the rotary screen apparatus can perform thickly embossed printing on a paper sheet with a special ink or the like, the rotary screen apparatus is used for the purpose of giving a high-quality looking and touch.
The screen of the rotary screen apparatus is commonly manufactured in the following way. Firstly, multiple small holes are formed in the entire body of a thin cylindrical base material both in the circumferential and the axial directions so that the base material has a mesh structure. Then, a photosensitive material is provided to all of the small holes so that all of the small holes can be filled up with the photosensitive material. The photosensitive material is removed from some of the small holes that are located at positions corresponding to a pattern. The screen thus manufactured is then used in a screen printing process for the pattern, and the photosensitive material is removed from the entire body of the base material after printing. Subsequently, the base material is provided with another photosensitive material with which all the small holes of the base material are filled up. Then, the photosensitive material is removed from some of the small holes that are located at positions corresponding to a different pattern. The screen thus manufactured is then used in a screen printing process for the new pattern.
As described above, the base material for the above-described screen can be used repeatedly. A screen of another type is provided with a base material that is cylindrical in shape and made of a thin plate with no holes formed therein. The screen is manufactured by forming small holes in the base material so that the small holes can be formed at positions corresponding to a pattern. Once the screen printing process is finished for the pattern using the screen, another screen has to be prepared from the beginning (e.g., the preparation for another base material). The screen with the repeatedly usable base material is much less wasteful than the one with the base material that is not repeatedly used. Accordingly, since the screen with the repeatedly usable base material can be used at lower costs, the screen of this type is widely used.
Incidentally, the screen is thin, and thus biased outwards by the squeegee in the radial direction. Suppose a case where the screen faces grippers of the impression cylinder that are provided for holding the paper sheet or a case where the screen faces a gap portion which is formed in the impression cylinder and in which the grippers are installed. In these cases, the contact of the screen with the grippers (projections) or the falling down of the screen into the gap portion may cause damages to the screen. As a result, the screen tends to have a shorter lifetime.
For example, a measure to address this problem is proposed in Japanese Patent Publication No. 2000-504643A and U.S. Pat. No. 5,960,716. In the disclosure, the grippers and gripper pads are provided in a gap portion formed in the impression cylinder to prevent the grippers and the gripper pads from projecting out from the outer peripheral surface of the impression cylinder. Moreover, a cover that can be opened and closed is provided to entirely cover the gap portion. When a paper sheet is held and released, the opening and closing operations of the cover are synchronized with those of the grippers. In this way, while holding and releasing a paper sheet is made possible, the screen is prevented from falling into the gap portion of the impression cylinder and from coming into contact with the grippers (projections). As a result, the screen is protected from damages on the screen, which might otherwise occur.
Another example of such measures is disclosed in Japanese Patent Publication No. 2005-254640A. In the disclosure, the grippers and gripper pads are provided in a gap portion formed in the impression cylinder to prevent the grippers and the gripper pads from projecting out from the outer peripheral surface of the impression cylinder. A partial cover member that covers only a part of the gap portion is provided. Accordingly, when a paper sheet is held and released, the grippers are projected from the outer circumferential surface of the impression cylinder. In this way, while holding and releasing a paper sheet is made possible, the screen is prevented from falling into the gap portion of the impression cylinder and from coming into contact with the grippers (projections). As a result, the screen is protected from damages on the screen, which might otherwise occur.
Such apparatus structures as those described in Japanese Patent Publication No. 2000-504643A, Japanese Patent Publication No. 2005-254640A, and the like, however, have a problem. When the screen transfers from a surface of the paper sheet to a surface of the impression cylinder, the screen is abruptly pulled outwards in a radial direction by a length equivalent to the thickness of the paper sheet by the squeegee. The same place on the screen always receives an impact, and the strength of the screen is more likely to deteriorate. Thus, the service life of the screen may possibly be shortened.
Particularly, the rotary screen apparatus described in Japanese Patent Publication No. 2000-504643A or the like has the following problem. A certain position of the screen is always rubbed against the gap between the cover and the gap portion in the rotational direction of the impression cylinder. The strength of the screen is more likely to be deteriorated further, and eventually, lifetime of the screen is shortened.
Such problem as mentioned above may occur not only in a case where thickly embossed printing is performed on a paper sheet with a special ink, but also, in a similar manner such as a case where liquid is supplied to a sheet held by an impression cylinder, with a liquid supplying member, through holes formed in the plate for the rotary screen apparatus. For example, such a problem may occur in a case of applying varnish to an entire surface of a paper sheet.